Optical fibers with graded index core are not only used as multimode fibers, but it is also known that single mode fibers with appropriately graded core can, for instance, have zero chromatic dispersion at a wavelength that is substantially longer (e.g., at 1.55 .mu.m) than the wavelength of zero material dispersion of the fiber material. Such "dispersion-shifted" single mode fibers are of considerable interest. In current practice the fiber material is almost invariably silica-based (typically more than 80% silica).
Very low loss dispersion-shifted single mode fibers can be advantageously used in long haul transmission systems, where they may permit repeater spacings of 50 km or more. However, dispersion-shifted single mode optical fibers are expected to find also significant use in applications that do not demand extremely low loss. For instance, optical fiber in the loop and distribution portion of the public switched telephone and data transmission network, and in so-called local area networks, frequently may require only moderately low loss. In these and similar applications, the distances involved typically are relatively modest, and repeaters may not be required, even if fiber having only modest loss (e.g., up to a few dB/km at the operating wavelength) is used. However, for obvious reasons, it will be necessary that such fiber have relatively low cost. It is thus important that techniques be developed that will make possible relatively low cost production of low to moderate loss graded index optical fiber.
All commercially significant prior art techniques for producing graded index fiber involve deposition of doped high silica material (typically the product of a reaction involving gaseous precursors such as SiCl.sub.4, GeC1.sub.4 and O.sub.2), with the dopant concentration in the deposited material caused to be a function of the radial coordinate, such that the desired radially varying index profile results. For instance, R. Yamauchi et al, Journal of Lightwave Technology, Vol. LT-4, No. 8, August 1986, pp. 997-1004 disclose a dispersion-shifted single mode optical fiber with a Gaussian core profile that was produced by the VAD process. Preform production was relatively complicated, involving the use of multiple burners and including the use of a special core deposition burner, and can be expected to pose significant repeatability problems. Furthermore, attainment of zero dispersion at the desired wavelength required selection of the elongated size of an as-grown preform and of the dimension of the overjacketing tube, steps which, inter alia, require costly measurements and, in a production mode, maintenance of a relatively large inventory of over-jacketing tubes and/or preforms.
In general, at least some currently technologically significant glass deposition techniques (e.g., VAD) can readily produce step index profiles but do not readily lend themselves to the production of graded index profiles.
Prior art techniques for forming graded index fiber generally were designed such as to prevent alteration of the as-deposited dopant profile. For instance, all flame hydrolysis techniques such as VAD and OVPO require removal of OH, water, and possibly other hydrogen-containing species from the porous body that is formed by deposition of the high-silica soot produced by the flame hydrolysis, and consolidation of the porous body. This removal, usually referred to as "drying", is generally accomplished by exposing the porous body to a chlorine-containing atmosphere while the body is maintained at an elevated temperature, with the conditions chosen such that loss of germania (or other dopants) from the porous body is minimized.
In particular, the prior art teaches that the drying and consolidation atmosphere should contain a relatively high proportion of oxygen, and that the chlorine content of the atmosphere be kept relatively low. See, for instance, U.S. Pat. No. 4,165,223, which teaches that the drying gas mixture should comprise chlorine and oxygen in amounts sufficient to substantially eliminate water from the preform while not removing excessive amounts of dopant oxide therefrom, also teaches that the amount of chlorine and oxygen should be within the ranges of 0.1 to 9 and 1 to 99.9 vol. %, respectively. More particularly, U.S. Pat. No. 4,165,223 teaches that, based on actual experience, the ratio (vol. % Cl.sub.2).sup.2 /(vol. % O.sub.2) should be less than 1 volume %, to prevent excessive leaching of dopant oxide from the preform, with the preferred value of this ratio being less than 0.1 volume %. Furthermore, U.S. Pat. No. 4,165,223 teaches that the preform pore structure should be substantially uniform, since this minimizes the tendency for oxygen to be trapped within the preform.
Among known techniques for making optical fiber are those which involve providing a glass core member, depositing on the core member porous material, consolidating the porous material such that a homogeneous glass body results, and drawing the optical fiber from the homogeneous glass body. See, for instance, U.S. Pat. No. 4,230,472, which teaches that the core member should consist of glass doped to have a relatively high refractive index, and that the particulate material deposited on the core member should have a composition such that the glass resulting therefrom has a relatively low refractive index, with the core of the optical fiber drawn from the body consisting of core member material and the cladding of the optical fiber consisting of the deposited material.
In view of the economic significance of graded index optical fiber, methods that can be used to economically and reliably produce such fibers are of substantial interest. This application discloses such a method.